Enhanced superconductivity and mixed-dimensional behaviour in infinite-layer samarium nickelate thin films

Rare-earth infinite-layer nickelates are emerging unconventional superconductors, with materials synthesis largely limited to early lanthanide compounds. Here, we report phase-pure samarium-based nickelate thin films on (LaAlO₃)_0.3(Sr₂TaAlO₆)_0.7 (001) substrates, including the first demonstration of Sm_1-xSr_xNiO₂. Co-doped compounds achieve a record-small c-axis parameter (3.26 Å) and superconducting transitions up to 32.5 K, revealing a clear correlation between decreasing c-axis parameter and increasing critical temperature across different rare-earth systems. Angle-dependent magnetoresistance shows a hybrid 2D/3D superconductivity with enhanced rare-earth 5d-Ni 3 d orbital coupling, confirmed by resonant inelastic X-ray scattering. In addition, increasing Eu concentration drives a shift toward 3D superconductivity, and Eu-containing samples exhibit distinctive negative magnetoresistance even in the superconducting state. These findings advocate clear materials design principles for higher transition temperatures and exotic physics in infinite-layer nickelate superconductors through structural engineering of the rare-earth site.